Technical Field
The present invention relates to a switching power-supply device that with a simple configuration is capable of suppressing wasteful loss and achieving higher efficiency in a wide range of load states, particularly light load states.
Background Art
A switching power-supply device generally includes a main switching element that controls current flowing in a primary coil of a transformer to which a DC input voltage is applied, and a voltage output circuit that generates a prescribed DC output voltage by rectifying and smoothing a voltage induced in a secondary coil of the transformer. The switching power-supply device is furthermore configured to include a switching control circuit for controlling the main switching element on/off in response to the input of a feedback signal based on the DC output voltage, thereby making the DC output voltage constant.
Switching power-supply devices can be roughly divided into self-excited and separately-excited types. The self-excited switching power-supply device is a type in which an on/off period of the main switching element is determined by a switching circuit portion itself performing oscillation operations. Meanwhile, the separately-excited switching power-supply device is a type in which an oscillator that determines the on/off period of the main switching element is provided externally.
Additionally, the circuit types of switching power-supply devices include a forward type and a flyback type, which have different ways of transmitting energy through a transformer. The forward type is a type that generates the DC output voltage by transmitting energy to the secondary coil side through the transformer when the main switching element is turned on. The flyback type, meanwhile, is a type that generates the DC output voltage by transmitting energy to the secondary coil side through the transformer when the main switching element is turned off.
Meanwhile, current flowing in the main switching element or a voltage applied to the main switching element is also set to a sinusoidal shape in order to reduce loss in the main switching element. Such a switching power-supply device is referred to as a current resonance-type switching power-supply device or a voltage resonance-type switching power-supply device. Incidentally, the current resonance-type switching power-supply device is provided with a resonance capacitor connected in series to the primary coil of the transformer, and controls the main switching element on/off at the timing when current flowing in the main switching element becomes zero (0). Meanwhile, the voltage resonance switching power-supply device is provided with a resonance capacitor connected in parallel to the main switching element, and controls the main switching element on/off at the timing when a voltage applied to the main switching element becomes zero (0).
Specifically, the current resonance-type switching power-supply device is configured as illustrated in FIG. 14, for example. In this switching power-supply device, two switching elements Q1 and Q2 connected in series are used as main switching elements. The switching element Q1 is connected in parallel to a primary coil P of a transformer T via a resonance capacitor Cr, and the switching element Q2 is connected in series to the primary coil P of the transformer T. The switching elements Q1 and Q2 are constituted of MOS-FETs, for example, and are generally turned on/off complementarily in response to a gate signal from a control circuit CONT implemented as an integrated circuit.
A DC input voltage Vin is applied to the primary coil P of the transformer T from a DC power source BAT, via the resonance capacitor Cr and the switching element Q2. The DC power source BAT rectifies a commercial AC power supply, for example, to generate the DC input voltage Vin. The DC input voltage Vin is then is smoothed through an input capacitor Cin and applied to the switching power-supply device.
Here, the switching element Q2 serves to supply current Icr to the primary coil P of the transformer T via the resonance capacitor Cr when on, and store energy in a resonance circuit, constituted of the resonance capacitor Cr and a resonance inductance of the transformer T. The switching element Q1, meanwhile, serves to discharge the energy stored in the resonance circuit through the primary coil P of the transformer T when on, and supply a reverse current Icr to the primary coil P. As a result, the current Icr flowing through the primary coil P of the transformer T has a sinusoidal waveform indicating a resonance arc.
Prescribed voltages are induced in secondary coils S1 and S2 of the transformer T by the current Icr flowing in the primary coil P of the transformer T. The voltages induced in the secondary coils S1 and S2 of the transformer T are subjected to full-wave rectification by diodes D1 and D2, and are then smoothed through an output capacitor Cout. In other words, the diodes D1 and D2 and the output capacitor Cout constitute a voltage output circuit that generates, from the voltage induced in the secondary coils S1 and S2 of the transformer T, a DC output voltage Vout to be supplied to a load RL.
Meanwhile, the DC output voltage Vout undergoes resistance division through resistors R1 and R2 and is detected as a detected voltage Vsens proportional to the DC output voltage Vout. An error voltage between the detected voltage Vsens and a prescribed reference voltage Vref set by a shunt regulator SR is supplied to the control circuit CONT via a photocoupler PC as a feedback signal. The control circuit CONT then makes the DC output voltage Vout constant by subjecting the periods in which the switching elements Q1 and Q2 turn on/off to feedback control on the basis of the feedback signal. A switching power-supply device configured in this manner is described in detail in, for example, Patent Document 1 and the like.